A common method for establishing connections between electrical components is solder bonding. Semiconductor chips are often joined to a printed circuit board (PCB) or each other in this manner. One typical solder bonding technique for soldering aligned arrays of contact pads on respective components includes the formation of similar bumps of solder on the contact pads of each array. The respective components are then arranged with the solder bumps oriented butt-to-butt and subjected to cycles of heat and pressure in a liquid or gaseous flux which melts the solder bumps to form the respective interconnections between the contact pad arrays.
However, conventional solder bump bonding is very difficult for contact pad arrays extending over large substrate areas, such as on the order of 5 mm.times.5 mm, wherein at least a region of the array contains closely spaced contact pads, such as less than 25 .mu.m between contact pad edges. In such bonding applications, it is difficult to align component surfaces containing the contact pads of the respective arrays exactly parallel to one another while applying bonding pressure. As a consequence, solder bumps in particular regions receive more bonding pressure then solder bumps in other regions. Such unbalanced pressure often causes excessive spreading of the solder bumps in the regions receiving more pressure resulting in electrical shorting between laterally adjacent contact pads.
In contact pad arrays employing larger pad separations, dam-like structures have been used around the contact pads to limit the expansion of the solder bumps during bonding. However, the use of such structures undesirably increases fabrication cost and complexity. Moreover, such structures are difficult to implement in closely spaced contact pad arrays such as those having separations of less than 25 .mu.m.
Also, conventional solder-bump bonding techniques present specific problems for joining contact pad arrays disposed on substrates having different coefficients of thermal expansion. Such problems arise when the substrates are subjected to the elevated temperatures used to melt the solder bumps. These temperatures often cause the substrates to expand by different respective amounts and upon cooling tend to break the solder bond junctions. Such breakage problems increase with reduced contact pad size and/or increased substrate size.
Thus, a need exists for a high yield solder bonding technique that enables bonding of large arrays of closely spaced contact pads and that is useful for joining contact pad arrays on substrates with different coefficients of thermal expansion.